You can run in salt water, but you must build your boat so that some of the aerial protrudes above the surface of the water when submerged as the radio signal will not penetrate the highly conductive salt water.
A chap in the States does this. He used a length of brass rod about a foot long- this means your depth is limited to under a foot, in reality this isn't that much of a handicap, most submariners operate close to the surface anyway.
In the case of your Gato, as it's a small boat, I would use a length of plastic tubing, and run the aerial wire up that. Cap the end of the wire off with a drop of 24 hour epoxy or silicone rubber. You will also need a waterproof exit for the receiver wire from your watertight enclosure. Again araldite or silicone will work. Whatever you do, make sure that the aerial length isn't extended or shortened beyond the original length, else the tuning of the receiver may be affected.
Finally, make sure your boat is equipped with a failsafe on the ballast tank and if a dynamic diver the throttle. This will help guard against the loss of your boat should you dip below the aerial length. I would also budget for a leveller as this will make running at what is effectively periscope depth, a much more refined experience.
